Conventional apparatuses for sensing the size of an original to be read use various schemes. An example is a method described in Japanese Patent Laid-Open No. 5-207239. In this method, the width of an original in the main scan direction is detected by illuminating the leading edge of the original with a light source and reading the reflected light from the original by a CCD. The width of the original in the sub-scan direction is detected by an original size sensor. The original size is sensed on the basis of these two reading results. This method detects the original size by using the CCD for reading originals to obtain the width in the main scan direction. Since this reduces the number of original size sensors, it is possible to realize inexpensive and efficient automatic original sensing.
In addition, a method described in Japanese Patent Laid-Open No. 9-329848 is known. In this method, the opening/closure of a cover plate for pressing an original to be read is detected and an original size is determined when the opening of the cover plate is detected.
Unfortunately, the above conventional techniques have the following problem. That is, in the above conventional original size sensing methods, original size can be basically detected, however, there is a possibility that the original size may be miss-detected since ambient light from, e.g., a room lamp other than the apparatus may be also read when reading the reflected light from an original, and this ambient light and the reflected light from the original can not be distinguished. In particular, the technique disclosed in Japanese Patent Laid-Open No. 9-329848 does not refer to the influence of ambient light.